The present invention relates to internal combustion engine superchargers.
Superchargers are frequently used on internal combustion piston engines to increase power of the engine. They are positively driven compressors used to pump air into the cylinders of the engine. The increase mass of oxygen forced into the cylinders by the supercharger allows the engine to burn more fuel, improving the volumetric efficiency and power of the engine. Superchargers are typically of a Roots (lobe) type, Eaton (twin screw) type or centrifugal type. Modern automotive superchargers are typically driven by a toothed belt from a toothed drive pulley on the engine's crankshaft, although they can also be chain, gear or shaft driven, or driven in some other manner. The loads on the belt can be quite high, as the supercharger can require as much as one third of the total crankshaft power of the engine. In present supercharger drive systems, the supercharger drive pulley and crankshaft drive pulley are both constructed of solid metal and allow no give to the belt.
When transitioning from higher throttle to lower throttle, and especially from full/high throttle to low/closed throttle, the rpm of the crankshaft decreases rapidly due to the decreased air flow. However, the inertia of the rotating mass of the supercharger acts against the supercharger rpm decreasing at a corresponding rate to the crankshaft rpm (depending on the drive ratio between the crankshaft and supercharger). This is particularly pronounced in high output supercharger applications, such as are found in racing applications, with very rapid transitions from full throttle, high engine (crankshaft) rpm to closed throttle, as might typically happen at the end of a drag strip run or when entering a corner. This is also pronounced on manually shifted cars that lift the throttle for each shift. This situation can create a very high reverse load on the belt, which can stretch, damage or even break the belt or cause other damage to the supercharger or supercharger drive system. Undesired loads, which damage or break components, or reduce their operating life, can also be caused even in more moderate situations which are not at the extremes of high output engines and/or very rapid full throttle/high rpm to closed throttle transitions. Many centrifugal superchargers overdrive the impeller as much as 6-1 to get impeller speeds in excess of 80,000 rpm, thus aggravating the amount of inertia the belt tries to stop when the throttle is suddenly closed.
Many superchargers used on street performance vehicles use serpentine belts for supercharger drive. The serpentine belts are longitudinally grooved to engage grooved pulleys. Such serpentine belts can slip on the pulleys. This can reduce loads on the supercharger and drive system and reduce belt breakage but this belt slippage also limits supercharger boost and thus, engine power. To counter this belt slippage, the width of the belts and pulleys can be increased but this again raises the peak loads on the supercharger and drive system and also absorbs more power from the engine to drive the wider belts. It is common to increase the width of the belts/pulleys from 6 grooves to 8, 10, 12, or even 16 groove belts to eliminate slippage. Many race cars use a 14 mm toothed belt to stop belt breakage but the use of such a belt over a standard 8 mm belt can absorb as much as 100 horsepower in a race engine, reducing total power output of the engine.